VEGETABLE CROPS

Potato Scab

Fact
Sheet Page 725.80 Date 10-1991

COOPERATIVE EXTENSION 
NEW YORK STATE  CORNELL UNIVERSITY

Rosemary Loria,
Department of Plant Pathology, Cornell University

Potato
scab is a common tuber disease that occurs throughout the potato growing regions
of the world. Although scab does not usually affect total yields, significant
economic losses result from reduced marketability of the tubers. Economic losses
are greatest when tubers intended for table stock are infected, since appearance
is important for this market. While superficial scab lesions do not greatly affect
the marketability of processing potatoes, deep-pitted lesions, however, do increase
peeling losses and detract from the appearance of the processed product. The occurrence
of scab and its severity varies by season and from field to field. Cropping history,
soil moisture, and soil texture are largely responsible for this variability.
Potato scab lesions can be confused with powdery scab, a disease caused by an
entirely different pathogen, the fungus Spongosporasubterranea (see
Cornell Cooperative Extension Information Bulletin 205: Detection of Potato
Tuber Diseases and Defects).

Symptoms and Signs

Potato
scab lesions are quite variable and distinctions have been made between russet
(superficial corky tissue), erumpent (a raised corky area), and pitted (a shallow-to-deep
hole) scab as depicted in figures 1, 2, and 3. All of these
can be caused by the same pathogen, Streptomycesscabies; however, the
type of lesion probably is determined by host resistance, aggressiveness of the
pathogen strain, time of infection, and environmental conditions.

Individual
scab lesions are circular but may coalesce into large scabby areas. Insects may
be involved in creating deep pitted lesions. The term "common scab" generally
refers to the response of the disease to soil pH. Common scab is controlled or
greatly suppressed at soil pH levels of 5.2 or lower. Common scab is widespread
and is caused by S. scabies. "Acid scab" seems to have a more limited
distribution, but has been found in several states in the Northeast. This disease
occurs in soils below pH 5.2, as well as at higher levels. The causal agent, S.
acidiscabies, is closely related to the common scab pathogen and can grow
in soils as low as pH 4.0. Acid scab is controlled by crop rotation, but can be
a problem when seed is produced in contaminated soils. Acid scab lesions are similar,
if not identical, to those caused by S. scabies.

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on Photo for Magnification (This feature is not currently available)

Disease
Cycle

Most if not all potato soils have a resident population
of S.scabies which will increase with successive potato or other
host crops. Scab-susceptible potato varieties appear to increase soil populations
faster than scab-resistant varieties. Rotation with grains or other nonhosts eventually
reduces but does not eliminate the S. scabies population. This pathogen
is a good saprophyte and probably reproduces to some extent on organic material
in the soil. Given the right environmental conditions and a scab-susceptible potato
variety, scab can occur in afield that has been out of potatoes for several years.

S.
scabies infects young developing tubers through the lenticels and occasionally
through wounds. Initial infections result in superficial reddish-brown spots on
the surface of tubers. As the tubers grow, lesions expand, becoming corky and
necrotic. The pathogen sporulates in the lesion, and some of these spores are
shed into the soil or reinfest soil when cull potatoes are left in the field.
The pathogen survives in lesions on tubers in storage, but the disease does not
spread or increase in severity. Inoculum from infected seed tubers can produce
disease on progeny tubers the next season.

The disease cycle
of S. acidiscabies is similar to that of S. scabies, but
the acid scab pathogen does not survive in soil as well as common scab. Inoculum
on seed tubers, even those without visible lesions, seems to be important in disease
outbreaks in New York.

Factors Influencing Disease Severity

 Varietal resistance. Though the mechanism of resistance
to scab is not well understood, varieties with different levels of resistance
have been identified through field screening programs. Using resistant varieties
is an effective tool for management of scab. Resistant varieties are not immune,
however, and will become infected given high inoculum densities and favorable
environmental conditions. The limited information available indicates that there
is a good correlation between resistance to common scab and to acid scab among
potato varieties. Consult potato disease-control recommendations for current information
on potato varieties with resistance to scab.

 Soil
acidity. Severity of common scab is significantly reduced in soils with pH
levels of 5.2 and below, but losses can rapidly increase with small increases
in pH above 5.2. Potatoes are commonly grown in soils with a pH of 5.0 to 5.2
for control of common scab. As mentioned, S. acidiscabies ("acid scab")
causes scab in low-pH soils. This species does not compete well with other soilborne
microbes, however, and can usually be controlled with seed treatments and crop
rotation.

While low-pH soils provide good control of common
scab, there are disadvantages associated with this management strategy. Plant
nutrients are most available at soil pH levels near 6.5. Since acid soils are
unfavorable for most vegetable and field crops, the number of them that can be
grown in rotation with potatoes is limited. Maintaining soils near pH 5.0 reduces
both fertilizer efficiency and minor element availability, and may result in phytotoxic
levels of some minor elements. Potatoes grown in soils near pH 6.5 produce higher
yields with less fertilizer. Lack of crop rotation aggravates many pest problems,
especially the Colorado potato beetle.

 Soil moisture.
Soil moisture during tuberization has a dramatic effect on common scab infection.
Maintaining soil at moisture levels above -0.4 bars (near field capacity) during
the 2 to 6 weeks following tuber initiation will inhibit infection by S.
scabies. Bacteria that flourish at high soil moisture appear to outcompete
S. scabies on the tuber surface. However, maintaining high soil moisture may be
difficult in some soils, and it is possible that other disease problems may be
aggravated by excessive irrigation.

 Soil type and
soil amendments. Light-textured soils and those with high levels of organic
matter are favorable to scab infection.Streptomyces are generally involved in
the decomposition of soil organic matter, and therefore thought to be stimulated
by its presence. Applying manure to potato fields can cause an increase in scab
infection. Coarse-textured soils are conducive to scab, probably because of their
moisture-holding capacity; thus, gravelly or eroded areas of fields that tend
to dry out rapidly are often sites of heavy scab infection.


Crop rotation. Crop rotation reduces the inoculum levels in potato fields,
but S. scabies can survive for many years in the absence of potato. This may be
due to saprophytic activity or an ability of S. scabies to infect
other plants. Infection of seedlings of many vegetables and fleshy roots of beet,
cabbage, carrot, radish, spinach, turnips and other plants has been reported.
Rotation with small grains, corn, or alfalfa appears to reduce disease in subsequent
potato crops. Red clover, however, stimulates problems with common scab and should
not be used in fields where scab has been a problem. S. acidiscabies
appears to have a host range similar to that of S. scabies but
does not survive well in the presence of nonhost crops.

Recommended
Disease-control Strategies

1. Useresistant
varieties in fields where scab is a problem

2. Use scab-free
seed and seed treatments to prevent introduction of the pathogen into fields.
Seed treatments do not eliminate the pathogen but will provide some suppression
of disease. Consult current potato disease-control recommendations for appropriate
seed treatments.

3. Rotate heavily infested fields away
from potatoes and alternate hosts such as radish, beets, and carrots. Use small
grains, corn, or alfalfa in rotations; avoid red clover.

4. Maintain
soil pH levels between 5.0 and 5.2 by using acid-producing fertilizers such as
ammonium sulphate. Avoid or limit the use of such alkaline-producing amendments
as lime and manure.

5. Avoid moisture stress during
the 2 to 6 weeks following tuberization.